Rijksuniversiteit Groningen MSc Astronomy Detectability of biosignatures in nearby terrestrial exoplanetary atmospheres with the ELT Supervisor: Author: Prof. dr. F. F. S. van der Tak A. H. Corporaal Dr. M. Min August 19, 2019 Abstract Biosignatures, features in planetary atmospheres that can be related to life and may be detected re- motely in atmospheres of potential habitable planets, play a key role in our current search for life beyond Earth. Using the knowledge of life to create these signatures, the possible abiotic origin and the sources and sinks in the atmosphere of these signatures, it can be deduced whether a planet may host life. Moreover, for characterisation of atmospheres, the concept of the habitable zone (HZ), the shell around a star where the planetary temperature could be such that liquid water could exist on the surface of a terrestrial planet, was used. Planets located in this region around the host star have preference in future studies related to the search of life beyond Earth. In this thesis it was investi- gated whether frequently proposed biosignatures gases oxygen (O2), ozone (O3), nitrous oxide (N2O), methane (CH4), ethane (C2H6), ammonia (NH3), and methyl chloride (CH3Cl) are detectable with planned instruments for the Extremely Large Telescope (ELT). We included the detectability of the habitability marker water (H2O) in our analysis. This was realised by building atmospheres using the exoplanet modelling code ’ARtful Modelling code for exoplanet Science’ (ARCiS) written by Michiel Min. Synthetic spectra for potentially inhabited planets are created using opacity tables at the high resolutions at which the instruments for the ELT will operate. Synthetic reflection- and transmis- sion spectra for the Mid-infrared E-ELT Imager and Spectrograph (METIS) and the High Resolution Spectrograph (HIRES) respectively are created assuming clear atmospheres. For investigating the de- tectability of biosignatures, the transmission of the atmosphere of the Earth is considered. We consider a hypothetical Earth-Sun analogue at 10 pc, Trappist-1 d, Trappist-1 e, Trappist-1 f, Trappist-1 g, LHS 1140 b, and K2-18 b as examples. It was concluded that METIS may be able to detect the O3 3.6 µm band, the N2O 3.7 µm band and H2O features in the wavelength range of 3.5-4.3 µm within 27 observation nights for Earth at 10 pc, Trappist-1 d, Trappist-1 e, Trappist-1 f, and Trappist-1 g. The other spectral features in the mid-infrared will be blocked by Earth’s atmosphere. For the wavelength range of HIRES, the optical and near-infrared, it was concluded that features will not be detectable. Acknowledgements Foist of all, I would like to thank my thesis supervisors, Floris van der Tak and Michiel Min, for their assistance during this thesis. You were available for questions when I needed help with concepts, analysis or understanding codes. Thank you for giving opportunities to develop myself with oral presentations and with research skills. I would like to acknowledge Migo Müller as the second reader of this thesis. Thanks for willing to read this thesis. I would like to thank the master students from room 0134 for the fruitful (related and unrelated) discussions, the motivation and the scientific support in good times and in tougher times. I would like to thank the Kapteyn Astronomical Institute in general for educating me, for a great ambiance and for being a place were I could develop myself scientifically. Last but not least, I want to thank my mom, my brother and my neighbours from number 64 for providing continuous encouragement and emotional support. Not only throughout this thesis but during my years of study, during my whole life, you have always been there for me. This thesis would not have been possible without you. 1 Contents 1 Introduction 4 1.1 Exoplanets...........................................4 1.2 Life...............................................6 1.2.1 Definitions of life....................................6 1.2.2 Habitability and the habitable zone.........................6 1.3 Biosignatures..........................................9 1.3.1 Gaseous biosignatures................................. 10 1.3.2 Surface biosignatures.................................. 17 1.3.3 Temporal biosignatures................................ 19 1.3.4 Antibiosignatures.................................... 21 1.4 Temperatures of planets.................................... 21 1.5 Goals and outline of this thesis................................ 22 2 Characterisation of planetary atmospheres 23 2.1 Detection techniques for exoplanetary atmospheres..................... 23 2.1.1 Transit method, secondary eclipses and phase curves................ 23 2.1.2 Direct imaging..................................... 25 2.2 The Extremely Large Telescope (ELT)............................ 27 2.2.1 Exoplanet Imaging Camera and Spectrograph (EPICS).............. 28 2.2.2 Mid-infreared E-ELT Imager and Spectrograph (METIS)............. 28 2.2.3 High Resolution Spectrograph (HIRES)....................... 29 2.3 The HITRAN molecular spectroscopic database....................... 30 3 Method 32 3.1 ARtful Modelling Code for exoplanetary Science (ARCiS)................. 32 3.2 The Habitable Exoplanets Catalog.............................. 36 3.2.1 The Earth Similarity Index (ESI)........................... 37 3.2.2 The Habitable Zone Distance (HZD)......................... 37 3.2.3 The Habitability Zone Composition (HZC)..................... 37 3.2.4 The Habitable Zone Atmosphere (HZA)....................... 38 3.2.5 The Standard Primary Habitability (SPH)..................... 38 3.2.6 Planetary Mass Classification (PMC)........................ 39 3.2.7 Planetary Thermal Classification (PTC)....................... 39 3.3 Simulated noise......................................... 40 3.4 Detectability.......................................... 41 2 4 Results 43 4.1 Simple isothermal atmosphere consisting of one gas..................... 43 4.1.1 Effect of the surface pressure............................. 44 4.1.2 Effect of the surface temperature........................... 46 4.2 Atmosphere with realistic P-T profile for one gas...................... 47 4.2.1 Effect of surface pressure............................... 49 4.2.2 Effect of surface temperature............................. 52 4.3 Atmosphere consisting of a mixture of gases......................... 52 4.3.1 Molecules with a constant mixing ratio throughout the atmosphere........ 52 4.3.2 Molecules distributed vertically throughout the atmosphere like Earth...... 75 4.4 Atmospheres and detectability for some known planets................... 80 4.4.1 HIRES.......................................... 82 4.4.2 METIS......................................... 85 4.5 Photon noise and Optimal resolution for characterising atmospheres with the ELT... 87 5 Discussion 89 6 Conclusion 92 6.1 Plans for the near future.................................... 92 3 Chapter 1 Introduction The concept of life beyond Earth has been around for centuries. This has led to the hope of finding inhabited extrasolar planets and identifying whether we are alone. This interest has brought us to find and characterise exoplanets around nearby stars. The first scientific paper related to the search of alien life was published halfway the twentieth century. In their paper Cocconi and Morrison (1959) suggested to look for radio signals around the 21 cm line of neutral hydrogen to hunt for intelligent life forms on planets orbiting nearby stars (within 15 light years). In that way, they suggested, we would be able communicate with those civilisations. Remotely detecting life was first proposed in the 1960s for Solar System planets (Lederberg, 1965). It was realised that the environment, including the atmosphere, on Earth is altered by the presence of life. This could therefore also have happened on other planets. A mature biosphere may change both atmospheric and surface properties of a planet and these may be remotely detectable because of the influence life may have on the atmospheric composition (Lovelock, 1965, 1975). To date, telescopes are planned and designed such that a biosphere may be remotely detectable. Before these telescopes are used to observe planetary systems outside the Solar System, a theoretical framework for interpreting the data is needed (Lovelock, 1975). One of the interesting questions that is tried to be answered in the exoplanet community is whether exoplanets host life. Not necessarily intelligent life is considered to hunt for but signs of all life forms are subject of modern studies. The first proof of planets around other stars than the Sun came in 1995 when 51 Pegasi b, a hot-Jupiter orbiting around the main-sequence star 51 Pegasi, was discovered by Mayor and Queloz (1995). The first discovery of an exoplanet, however, came three years earlier when Wolszczan and Frail discovered exoplanets orbiting the pulsar PSR 1257+12 (Wolszczan & Frail, 1992). The first Earth-sized planet located at such a distance from the star that it is located in the Habitable Zone (HZ) of that star was discovered in 2014 (Quitana et al., 2014). They reported the discovery of Kepler-186f transiting an M-dwarf. 1.1 Exoplanets The search for and characterisation of extrasolar planets, or exoplanets, planets orbiting other stars than our Sun, are of importance for the goal and interest of finding life elsewhere. Finding life goes beyond the detection of exoplanets. With 4106 confirmed exoplanets identified in August 20191, the field is moving from the